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1. Field of the Invention
The present invention relates generally to techniques for sensing and adjusting the position of a movable element, and more particularly of a pivoted armature reflector for steering optical beams at high speed and with high precision through sequences of positions including dwell intervals of constant position.
2. Description of Prior Art
There are many applications for steering an optical beam with high speed and precision between angular directions in either a random or a predetermined sequence. Agile beam steering in two dimensions permits successively illuminating targets in a number of locations to reflect the beam for subsequent detection by an optical receiver.
Agile tuning of lasers require sensors with a high speed-accuracy product. High accuracy is achievable with charge-coupled-devices (CCDs) and interpolation but readout is slow. On the other hand, position sensitive detectors (PSDs) and capacitive position sensors easily achieve the speeds required of the fastest pulse lasers but suffer long term accuracy and stability. Since tuning accuracy requirements are absolute ones, system configurations must be selected to achieve the tuning accuracy in spite of the slow speed sensors.
Tunable lasers typically include an intracavity diffraction grating and a rotatable mirror. The wavelength of such lasers is tuned by adjusting the angle of incidence of the laser cavity beam against the diffraction grating. Such intracavity tuning requires very high accuracy and stability. Tuned CO2 lasers, for instance, require an angular range of typically 0.2 radians and an accuracy of 10 xcexcradians. This represents a range to accuracy ratio of 20,000. In some cases, and at lower accuracy, an optical parametric oscillator (OPO) material (usually a crystal with highly non-linear optical properties) is used.
In the crudest implementations, grating reflection angles have conventionally been adjusted by grating alignment micrometer screws. Manual micrometer adjustment is slow and severely restricts beam steering agility. Automated micrometer adjustment by stepper motor is hindered by the angular momentum of the diffraction grating, backlash, screw friction, and other forces.
Laser radar (LIDAR) systems can be used to transmit different wavelengths of light into airborne suspensions (such as smog or poison gasses) which have differing reflectivities or absorption to different wavelengths. The reflected light intensity is then measured for remote spectrographic analysis of suspension samples. In remote spectroscopy LIDAR applications it is advantageous to maximize the stability and repeatability of the output at each different wavelength. On the other hand, it is very advantageous to minimize intervals between transmitting wavelengths in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting different wavelengths at the laser""s maximum cyclic rate.
LIDAR system laser formats can consist of single wavelength high power pulses in a sequentially repeating pattern of wavelengths. Another class of applications require a pulse burst of multiple pulses at a given wavelength, periodically repositioning to the next wavelength for another pulse burst. Alternately the tuning/positioning requirement is for a dwell time of a CW (continuous wave) laser periodically repositioning for repeating dwell times of CW operation at other wavelengths. Thus, several types of laser formats have been used to sequentially transmit multiple wavelengths; single pulse, single wavelength pulse burst and CW. The latter two require a dwell or hold time at each wavelength.
FIG. 1 shows typical timing of these formats. Waveform 40a shows single pulse timing with a pulse at wavelength xcex0 followed by one at wavelengths xcex1 and xcex2 finally repeating xcex0 for a continuous pattern of three wavelengths. Dwell time for this case can be quite short. Waveform 40b is pulse burst laser timing where a burst of pulses at wavelength xcex0 is output, separated by a retuning time for the next pulse burst at wavelength xcex1. Finally waveform 40c is a CW laser output at wavelengths xcex0, xcex1 and xcex2 separated by the retuning intervals. Tuning stability for the pulse burst and CW lasers must be maintained during the dwell times, typically shown as the interval between times 41 and 42 in FIG. 1. The present invention is aimed at systems requiring a significant dwell but has applications for the single pulse type as well.
FIG. 2 and FIG. 1, positional waveform 40d, illustrate an example of prior art which can provide limited multiwavelengths with dwell times. Here a separate laser is used for each wavelength. In this example, laser 27 at wavelength xcex0, laser 29 at wavelength xcex1 and laser 31 at wavelength xcex2 are selected to the output beam 25 by flip mirror 33 driven by actuator 35 and conventional servo or stepper 37. Actuator 35 can be a low accuracy device since it simply controls the pointing of the output beam. The individual laser output power and stability are set by the accurate internal optics of lasers 27, 29 and 31. This approach suffers from the high complexity and poor flexibility of having an entire laser for each wavelength, since there are upwards of 100 possible wavelengths in the CO2 lasing spectrum. Chemicals may not be detected very efficiently with the limited repertoire of wavelengths that practicality dictates. In order to reliably detect a single chemical, 3 to 10 wavelengths may be required and other chemicals use their own unique set of wavelengths.
In FIG. 3 of the prior art, an intracavity conventional servo implementation is shown. Intracavity beam 74 is tuned by rotating mirror 76 which responsively changes the incidence angle of grating 72 thereby selecting lasing wavelength. Mirror 76 is rotated by actuator 78 in response to control by servo control 82. Mirror position is sensed by position sensor 79 then combined with desired position data 81. Servo control 82 provides output drive to actuator 78 responsive to the desired position data 81 and the actual position information from position sensor 79. FIG. 1 waveform 40e shows the required angular function. For CW or pulse burst tuning of CO2 lasers, mirror 76 must have sufficient accuracy from time 41 to time 42 to satisfy the power stability requirements of the laser or approximately 10 xcexcradians. The prior art is replete with high performance servo techniques for servo control 82 and sensors for position sensor 79. In order to enhance speed and stability, the servo techniques aim to keep the actuator drive high as it approaches its final position and to compensate for the delays inherent in sensors and actuator/load inertias. These techniques, to name a few, involve lead-lag networks, phase compensation, error integration, gain switching, open loop/closed loop switching, dither, observer models, velocity and acceleration feedback and mode switching. Conventional servos, to date, have been unable to satisfy either the single pulse applications or the CW/pulse burst applications for CO2 LIDAR systems. Although such arrangements as FIG. 3 can easily satisfy either the speed or accuracy required by the typical laser tuner, limitations prevent satisfying both. Sensor 79 implemented with PSDs or other high speed position sensing techniques are adequately fast but lack the required accuracy for tuning lasers. FIG. 1 waveform 40i illustrates the positional drift. When sensor 79 is implemented using interpolated CCD technology for adequate accuracy, none of the servo speed enhancements satisfy the 200 Hz and up tuning speed requirements.
FIG. 4 of the prior art represents related U.S. Pat. No. 5,450,202 which discloses an adaptive resonant positioner capable of single pulse tuning and with some capability for CW or pulse burst tuning. As disclosed, high accuracy and speed are combined as a result of the adjustment of adjacent pairs of drives on a pattern delayed basis. Mirror 66 position is sensed by pulsed reference laser 92 and CCD 98 in sensor 52 in response to timing pulse 80 and sensor processing and interpolation 114. Actual sensed position 88 is used via drive control 54 together with pattern information 198 to drive resonant actuator 56 via drive line 90 to substantially stop at precise positions for firing the pulsed laser, not shown. This results in accurately tuned adjacent laser firing angles separated by angular half periods of damped sinusoidal motion. In a first, pulsed burst/CW mode, multiple patterns each consisting of a single wavelength are used. Since adjacent laser firing angles are separated by sinusoidal transitions of zero amplitude in this case, tuning to that pattern""s wavelength is continuous. Sequentially switching single wavelength patterns then result in an equivalent multiwavelength pattern. FIG. 1 positional waveform 40f, represents this approach, where pattern switching is typically shown by the time interval 42 to 43 and the tuning dwell time typically by interval 44 to 45. Tuning will be invalid for an interval after pattern switching, typically shown by interval 43 to 44, while the new pattern is adapted or learned. The time interval 42 to 43 is xc2xd the actuator resonance as disclosed. A degree of success has been achieved with this technique but it is limited by the learning and pattern switching speed and thus the sequencing of laser wavelengths of hundreds of hertz is difficult.
The related U.S. Pat. No. 5,450,202, in a second and faster pulsed burst/CW mode, has sequential wavelength tuning with dwell intervals shown by FIG. 1 positional waveform 40g. In this mode, the agile tuner executes a single pattern which contains two or more adjacent identical wavelengths for each dwell interval required. In theory, time intervals typically shown as 41 to 42 would again be sinusoids of zero amplitude. In practice, even though the actuator including its load may be well characterized, a number of factors cause the angular function between the tuned wavelength positions to depart as shown exaggerated by dashed line 40h. These factors are excited by the transition drive in between dwell intervals and include among other things torque nonlinearities, hysteresis, parameter thermal variations, damping uncertainties and resonance frequency errors. Thus, tuning in the general case, is not accurate enough between the firing positions under control of the resonant tuner feedback loops. This technique is notably faster in that wavelength switching is accomplished in only xc2xd of a resonant interval shown typically as the interval 42 to 44. Although tuning might be adequate for OPOs, for instance, it would be difficult to characterize the actuator/load sufficiently accurately to reach CO2 laser tuning accuracy.
Conventional optical element positioning systems have been unable to perform accurate high rate optical beam tuning for sequentially addressed CW or pulse burst lasers requiring optical elements in the cavity to be stationary during the active dwell time and then to jump quickly and precisely to the next position in a sequence.
In accordance with the present invention, an agile positioner comprises a load position sensor, a load driving actuator and a drive sequencer means for driving the load rapidly and accurately to a desired position by isolating, in time, the sensing and drive functions.
It is a primary objective of the present invention to rapidly position an actuator for dwell periods with high precision suitable for CW lasers and for pulse lasers requiring bursts of pulses at a constant wavelength.
It is another objective to enable an adaptive resonant positioner to track dwell times more closely.
It is another objective to provide improved positioning speed while utilizing accurate but slow speed position sensors.
It is another objective to allow the combination of high accuracy feedback loop, high transition speed and slow speed sensor.
It is another objective to further improve positioning speed and accuracy by utilizing adaptive algorithms when wavelengths repeat in patterns.
It is another objective to better utilize the torque of an actuator.
It is another objective to control the shape of the transitions.
It is another objective to position non-linear optical crystals.
It is another objective to successively reposition an optical reflector to steer a light beam in desired directions.
It is another objective to provide an improved laser tuner.
For a given technology, cost and complexity, sensors follow a law similar to the Heisenberg uncertainty principle of atomic physics. For sensors, the more accurately the position is measured, the less may be known about where it is at the end of the measurement. In dynamic closed loop operations, long sensor delays cause instabilities which must be dealt with in either the frequency or time domains, invariably by slowing down the overall acquisition speed. The effect gets worse as the required accuracy increases because the loop gain must increase. In a sense the problem stems from trying to coordinate the delayed position information with the need to drive the actuator efficiently without really knowing where it is. The typical result is a severe reduction in drive as the target position is approached so that periods of overshoot or undershoot can be reduced or prevented. The present invention, isolates the sensing delay from the drive execution by performing them separately with sensing occurring at a time when the velocity is substantially zero. In addition, motion of the actuator, once started, occurs in pure open loop fashion without regard to sensor output. This type of operation is made more effective by the highly predictable nature of some actuators such as galvanometers in low friction environments. Galvanometers can have repeatabilities below 1 xcexcradian and high torque predictabilities.
Preferred embodiments of a positioning system according to the present invention includes sensor means, drive sequencer means and actuator means. Actual positions of an optical element or a mirror are sensed and resolved by sensor means preferably including lighting means for emitting light to be reflected by the mirror, optical detector means for detecting the intensity distribution of the reflected light and producing an intensity sample waveform and interpolating means for resolving sample waveform values to provide interpolated actual position values. The actual position values, measured when actuator means is substantially stopped, are fed to a drive sequencer means preferably including memory means for storing desired position information including possible dwell intervals of constant position and for storing drive control information, error determining means for determining differences between the desired and actual mirror positions at given times and open loop sequence computing means for modifying and storing adaptive and non-adaptive drive control sequences for later use to control an actuator to reposition the mirror closer to the desired positions. Open loop sequences as used in this specification are torque or force functions of time, meeting boundary conditions of position and velocity, varying in amplitude, timing and/or polarity but not affected by position measurements once started.
In a first preferred embodiment, a drive sequencer means is used to modify the actuator drive of an adaptive resonant positioner as disclosed in related U.S. Pat. No. 5,450,202 or an equivalent positioner. The adaptive resonant positioner, operative on a resonant load, provides a sensor and drive control capable of positioning an actuator including low accuracy dwell intervals of constant position as described in the prior art of FIG. 4. The positioner is modified to provide one or more additional actual position signals in between resonant points normally required for operation of the resonant positioner. These additional actual positions are used by the drive sequencer to compute and adapt to provide improved drive information to cause the actuator to more closely track a constant position during periods of dwell programmed into the resonant positioner by repeating the same desired position in successive positions of the pattern. The improved drive information is an open loop sequence format and is algebraically added to the actuator drive provided by the resonant positioner in such a way that the positioner""s normal pulse laser operation is substantially unaffected at the points of loop closure but that closer tracking of the desired position occurs in the interval between. Boundary conditions on the open loop sequence are that no net position or velocity change appears at the normal resonant positioner lock points.
In a second preferred embodiment, a non-resonant actuator is sensed by a position sensor to provide actual position signals. A drive sequencer receives real time desired position data or data in the form of a pattern to be repeated and compares actual position data with desired position data. It computes open loop sequences of drive for positioning during transition and dwell times of the input pattern. Open loop sequences are computed based on position errors and an accurate model of the actuator and load. Loop stability at high speed is achieved by performing position sensing when the actuator and load are substantially stopped, maintaining an accurate model of the actuator and load and functionally ignoring the sensor output while moving the actuator in its open loop moves. Open loop sequences are applied to a non-resonant actuator which preferably has the characteristics of high repeatability, low friction and predictable torque characteristics for stability and rapid positioning. Operation generally proceeds in positioner cycles consisting of a position measurement followed by an open loop sequence move. Boundary conditions on the open loop sequence moves are that position change equal the position error and final velocity equal zero.
A third preferred embodiment has the basic form of the second embodiment except that the desired position data is always in the form of a fixed or slowly changing pattern to be repeated and one or more of the positioner cycles within a position of the pattern is adaptively modified for more rapid transition between positions of the pattern. Adaptive open loop sequences are computed based on the drive just used, current position error and an accurate model of the actuator and load. Adaptive sequences are executed on the next pattern. Boundary conditions are the same as for the second embodiment.
Among the advantages of the invention is that accurate but slow speed sensors, such as CCDs, can be used to determine fast open loop moves for a tight loop without introducing loop instabilities. Interpolated CCD sensors, for instance, can have pulsed illumination resulting in equivalent sensing aperture bandwidths in the MHz range, but when used in conventional feedback systems, readout delays cause closed loop bandwidths in the kHz range. The invention also has utility in less accurate positioning applications which require extremely high repositioning rates. Another advantage is that open loop moves allow for setting actuator transition rates and shapes independent of stability. Once locked onto a repeating pattern of relatively unchanging desired positions, the invention can successively reposition and stop the armature at actual positions including dwell times of constant position within, for example, 1 part in 20,000 of the desired positions. These and other objects of the present invention will become apparent to those skilled in the art upon reading the following detailed disclosure of the preferred embodiments as shown in the several views of the drawing.